a narrow win or loss for Biden's seen as a problem for the Dems and requires a period of reflection, but a narrow win or loss for Trump validates him and the GOP can just carry on as normal
there's a couple things there
GOP establishment is yoked to Trump - both personally and as the embodiment of populist backlash - so they don't have any choice but to go all in on him to the bitter end, whatever that is. I'd recommend
@craner's Palin essay (and his and my follow-up comments) for anyone who wants to go into that in more detail, but the GOP has been playing with this fire for decades and it finally jumped out of their control, the question is to what extent that fire can outlast Trump personally.
otoh for Democrats the relative closeness of the election - the popular vote is a clear Biden victory w/exactly the same margin as 2016, but it's still a 51-49 kind of victory, not a landslide - makes it clear 2016 was not an aberration. the question again is to what extent that's due to Trump personally/general discontent w/American ruling class vs actively preferring GOP. the one thing I think the DNC etc should really be troubled by is Biden's struggles with Latino voters outside Florida. the GOP has been trying to offset demographic change by somehow attracting Latinos without losing its nativist base, and if its managed to square that circle - or it turns out more assimilated Latinos don't care as much about immigration etc - then it's potentially a real problem for the idea that demographic change almost universally favors the Democratic Party.